The problem of producing duplex copies from duplex originals (i.e., duplex-to-duplex copying) has been addressed in the prior art.
U.S. Pat. No. 4,099,150 discloses a copier having a last-in-first-out (LIFO) duplex tray (51), and a document feeder having upper (57) and lower (63) original document trays. The upper tray is a LIFO tray, whereas the lower tray is a first-in-first-out (FIFO) tray.
A multi-page duplex original document is placed in the document feeder's upper tray, face up (FIG. 4)--i.e., with page 1 facing up. The document is now inverted by feeding the document out of the upper tray, through the copier's exposure area (70) without copying, and into the lower tray (FIG. 5).
Copying now occurs as sheets are fed out of the bottom tray, into the exposure area, and back to the bottom tray. One copy set is made for each circulation of the original document, and the copy set(s) are placed in the duplex tray, odd pages facing down.
Now it is necessary to circulate the original document from the lower tray to the upper tray (FIG. 7), and then from the upper tray to the lower tray (FIG. 8). Only then is the copier ready to place the odd numbered pages on the blank sides of the sheets now residing in the duplex tray.
Again, the original document is circulated through the exposure area once for each copy set.
When finished, the original document resides in the document feeder's lower tray, face up, and the copy sets reside in the copier's exit tray (53), face up.
U.S. Pat. No. 4,140,387 again shows a document feeder having an upper (32) and a lower (33) original document tray. Both of these trays are FIFO trays.
The copier does not include a duplex tray. Rather, a single-sheet-turn-around-device (110) operates to reverse a side-one copy sheet, and then immediately returns the sheet for side-two copying.
The original duplex document is placed in the upper tray, odd pages facing up (FIG. 4). The document is now fed to the bottom tray, where it now resides even pages facing up, but in increasing page sequence (i.e., pages 2, 4, 6 for a six page duplex document). Therefore, the original document is now in scrambled page order, i.e., the page order is not correct when progressing through the document from either direction.
Now copying can begin. The highest numbered even page (i.e., page 6) is first copied. This copy sheet is sent to the turn-around-device (110 of FIG. 5), from where its blank side is returned for copying of the highest numbered odd page (i.e., page 5)--as the document feeder has reversed the original document sheet to present the highest numbered odd page (FIG. 4).
This copy sheet is then placed in the copier's exit pocket (105) with the highest numbered odd page facing up.
This process repeats until all original pages have been copied--and again repeats for each copy set to be produced.
All of these copy sets reside in the exit pocket face up. The original document must be circulated to the document feeder's upper tray before it is restored to its correct page order, face up.
U.S. Pat. No. 4,278,344 discloses a FIFO document feeder (20) in which the original document stack is loaded face up into a tray (22), i.e., page 1 facing up. These sheets are bottom-fed, without inverting, to the copier's document glass (23). Here the even-numbered pages of the original set are copied (once for each page) in descending even-page order, and the copies thereof are stored in the FIFO duplex tray (108) image side down. The document sheets are then inverted and restacked in the document feeder tray, but with the even-numbered pages now facing up and in descending even-page number from the top to the bottom. However, the sheets of the original set are now in scrambled page order.
Now, the document set is circulated to the document glass, with sheet inversion, as many times as is necessary to make the requested number of copy sets--as the even numbered pages are copied, one at a time, from the highest numbered even page to the even page number "2".
These copies are also stacked in the duplex tray, even numbered pages facing down, and in ascending number order. After side-one of all of the requested sets have been accumulated in the duplex tray (note that the highest request number is limited by the maximum number of sheets that the duplex tray can hold), the document feeder feeds one original set with sheet inversion, such that the highest-numbered odd page is copied onto the blank side of the duplex-tray sheet which has the highest numbered even page on one side thereof--and so on until the first duplex copy set has been made.
As the first copy set is thus made, the original documents are returned to the document feeder tray inverted. At this time, the original set has been restored to its proper attitude, i.e., with the lowest numbered odd page face up.
Now the document feeder recirculates the original set, without inversion, as many times as needed to clear the duplex tray.
In the end, the original set resides face up in the document feeder tray, and the requested copy sets reside in the output (122 or 128) face up.